Issue: July 30, 2005

Irrigate old trees in rock landscape

Question:

Even though it rained a lot this spring, I decided to replace my lawn with gravel. I retained only two trees that shade my house. Friends said that my trees will die because of the change. Is that true? Will the gravel kill my trees? What can I do?

Answer:

Many people have converted to gravel landscapes in attempts to conserve water. It is true that gravel uses less water than grass, but the savings may not be as great as expected. Cooling costs and water usage in evaporative coolers may increase to do the cooling that plants in the landscape once did. Trees are often retained to shade homes and cool the landscape as a result. These trees require irrigation and reduce the water savings expected. Since the trees are often not considered during the landscape conversion, dying trees have been observed as a result. There are several reasons for this.

In converting from lawn to gravel, the grass is often dug or scooped from the landscape. Tree roots, which have been sharing the soil and water with the grass, may be damaged in this process. If the roots are damaged too severely, the trees will be injured or die. Both results create a hazardous situation in the landscape as branches or whole trees become susceptible to falling on your house, vehicles, or even you. Watch for dieback associated with root damage, removing dead or severely injured branches as the damage becomes apparent.

If the grass was removed by appropriate chemical treatment, the tree roots may not be injured. However, if the water needs of the trees are not considered, tree injury or death will be observed. Trees adapted to the lawn watering will quickly begin to show symptoms, even if there is rain. Rainwater harvested from the roof and directed to the root zone of trees will help, but a transition irrigation system may also be needed until the tree adapts to the new environment. During periods of drought, a system of providing water to the tree roots will also be needed. This may be provided by a garden hose and soaker hoses.

As you water trees, remember that the majority of their absorbing roots are found beyond the end of the branches, not near the trunk. The zone beyond the tree's branches (beyond the drip line) should be moistened to a depth of 2-3 feet twice a month in the summer, and once a month during the winter unless natural precipitation provides adequate moisture.

Another possibility is to install beds of low-water-demand flowering plants (xeriscape plants) in the areas beyond the trees branches where the xeriscape plants can share water with the trees. In this manner you can conserve water, use harvested rain water, and maintain the cooling that trees and other plants provide a shade trellis if you desire.

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico
State University's Cooperative Extension Service. New Mexico State
University is an equal opportunity/affirmative action employer and educator.